It is known that bitumen can be produced, in particular, from crude petroleum by distillation and deasphalting of the heavy fraction obtained by such distillation or distillations. Depending on the origin of the crude, bitumen consists of varying proportions of paraffinic oil (aliphatic or naphthenic), aromatic oil, resins and asphaltenes (on this subject, attention is directed to European Patent [EP] 246 956 and its U.S. equivalent, U.S. Pat. No. 4,810,367, both of which are owned by Applicants' assignee and are incorporated herein by reference).
Depending on its composition, bitumen can have a more or less thermoplastic character. It softens on heating, and its softening point, commonly referred to as its "ring-and-ball" temperature after the method of determination used (AFNOR.sup.1 method NFT 66008), can vary from about 30 to about 130 .degree. C., FNT .sup.1 Association Francaise de Normalisation=French Association for Standardization depending on the process by which the bitumen was obtained.
These temperatures may be insufficient for certain uses because of:
--risk of high temperature flow when used as sealing coatings; PA1 --inadequate elastic behavior; and PA1 --inadequate heat resistance; for instance, in certain cements used for certain applications (automobile chassis).
Among the techniques for improving the heat resistance of bitumen are, in particular, the following.
1. Oxidation of bitumen by the air-blowing method at about 250 .degree. C. However, the product thus obtained has disadvantages such as a high "ring-and-ball" temperature and is harder and more brittle.
2. Modification of bitumen by incorporation of certain polymers, which affords products with a "ring-and-ball" temperature as high as 120.degree.-150 .degree. C. However, these products have disadvantages such as that at a temperature of 150 .degree. C., such products are only slightly elastic.
3. Modification of bitumen by incorporation of a polymer such as SBR (styrene-butadiene rubber) or SBS (styrene-butadiene-styrene) which can be vulcanized with sulfur (or a sulfur donor) or a peroxide. However, this modification has disadvantages. It requires elevated and accurately controlled temperatures over a relatively long period of time. Further, the product thus obtained has a very high viscosity and is not readily compatible with application methods used by the industry. Also, vulcanization imparts to it an irreversible character which is not conducive to easy use. The product must be used within weeks after its production because its viscosity must remain compatible with that with which it will be used.
In the course of their research studies, Applicants have developed bitumen-polymer blends which avoid the various disadvantages and drawbacks presented by the products obtained by and of the methods heretofore described above.